Fighters from the Islamic State group parade in a commandeered Iraqi armored vehicle down a main road in the northern city of Mosul, Iraq. The fighters have held Iraq's second-largest city since June 2014 and it now serves as the group's stronghold in northern Iraq. The Associated Press

BAGHDAD — For weeks, U.S.-backed forces have been fighting to oust the Islamic State from key areas of northern Iraq in a series of small-scale battles that could have an enormous impact on the group’s “caliphate.”

A major prize in the clashes is a highway that serves as a lifeline for the Islamic State. It runs from the group’s Iraq stronghold in Mosul to its enclaves in northeastern Syria, including its self-styled capital, Raqqa, 300 miles away.

The battles are occurring as the Islamic State is causing growing alarm internationally over its brutal actions, which have included the murder of a captured Jordanian pilot and the beheading of 21 Egyptian Christians by Libya-based adherents of the extremist group.

In late January, however, Islamic State fighters suffered a setback as Iraqi Kurdish forces seized a stretch of the key highway at the town of Kiske, west of Mosul.

The Islamic State is still using the highway, detouring onto back roads to get around Kiske. But if the Iraqi Kurdish fighters can maintain and expand their hold on the road, the Islamist extremists “will be under a kind of siege in the area. It will be very hard for them” logistically, said Hisham al-Hashemi, an Iraqi researcher who is an expert on the radical group.

Blocking the highway would pressure the Sunni fighters to rely on lengthier and potentially riskier routes to transport people, cash and weapons, analysts say.

The road has been controlled by the jihadists since last summer. The peshmerga fighters from the semi-autonomous Kurdish region in northern Iraq launched the series of battles in December. The operation initially targeted Sinjar — an Iraqi town bisected by the crucial highway. But after fighting there stalled, Kurdish forces broadened their offensive.

The Islamic State had seized Mosul in June, annexing the Iraqi territory to Syrian terrain already under the group’s control. That month Islamic State leader Abu Bakr al-Baghdadi declared a caliphate over the thousands of square miles of land it had seized.

The loss of the highway between Mosul and Raqqa would be not just a logistical defeat for Islamic State but a psychological one, analysts say.

“The integrity of the caliphate — it’s built on continuous military victory,” said Jessica Lewis McFate, research director at the Institute for the Study of War, a Washington-based think tank.

The “caliphate” could lose legitimacy in the eyes of its supporters if it is unable to defend the land it has taken, she said.

Without their vital supply line, Islamic State operatives would be vulnerable in Mosul, which is increasingly isolated as Kurdish forces close in. They would also have to use alternative supply routes to Raqqa that meander through the harsh desert or expose them to dangers such as airstrikes by the U.S.-led coalition or confrontations with Iraqi security forces, analysts say.

If Islamic State convoys can no longer follow the main highway to Raqqa, which crosses the border at the al-Hasakah district in Syria, they could be forced to travel to the next-closest crossing they control, located at al-Qaim, 260 miles away.

But coalition warplanes have been raiding the al-Qaim area.

The corridor from Mosul into Syria isn’t the only Islamic State supply line that is under pressure. In Iraq, a separate route linking the western town of Haditha with the oil-producing town of Baiji and continuing north to Mosul could now also prove dangerous for the group as Iraqi security forces make gains in these areas.

If the fighters in Mosul can’t stay connected to Islamic State territory in Syria, “they will lose their claim that they have a state,” Hashemi said.

“They wanted to destroy the [Iraq-Syria] border,” he said. But if they can’t move back and forth between their state’s two most important regions, “they will have failed to destroy it.”

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